The performance of a fiber optic communication system is dependent on the strength of a signal that can be transmitted along a length of optical fiber in an optical communication channel and retention of the quality of the signal during transmission. A signal that has lost a portion of its strength during transmission along one length of optical fiber must be boosted to recover that strength before transmission along further successive lengths of optical fiber, or else the signal will be too weak to be detected and understood after transmission along several lengths of optical fiber. Similarly, the quality of the signal must be retained if it is to be clearly detected and understood after it has been transmitted along several lengths of optical fiber. Consequently, there is a need to monitor both the power and the quality of data transmitted as light signals along optical communication channels.
The number of optical fibers for communications has increased substantially in recent years, with the consequence that the costs for installation and operation of optical fiber systems have dropped to values comparable with those for coaxial cable systems. Thus there is a need for improved capability to monitor optical fiber systems.
Stephen G. Bavington has described the current state of the art optical fiber monitoring technologies in “Monitoring Fiber Networks”, published in the 2nd quarter, 2001 issue of Spirent.com e-magazine published by Spirent Communications. This article describes a system in which the apparatus for monitoring fiber optic communications includes single fixed optical taps on each fiber communications channel among an array of fiber communications channels. Each tap is in optical communication with a matrix of optical switches, which in turn is connected to selected signal analysers. In this system, each tap is a fixed fiber optical signal splitter with a fixed tap ratio.
Typically, electro-optic technology is used in systems for monitoring optical fiber communications. An optical signal being transmitted along an optical channel is intercepted and converted to an electrical signal for retiming and data recovery. The power and quality of the signal is assessed, and the performance of the optical fiber is thereby evaluated. This method of monitoring data communications is expensive. The cost of termination of fibers also is expensive. Thus the cost of monitoring the performance of optical fiber communication systems is increasing as the number of optical channels increases. Frequency and wavelength multiplexing allows each fiber to carry several signals simultaneously, thus further increasing the complexity and cost of monitoring optical fiber communications. Considerable effort is being made to enable monitoring of optical transmission of data, and methods of tapping said optical transmissions, as described in the following examples.
Chang-Hasnain et al. in U.S. Pat. No. 6,233,263, issued in 2001, describe a monitoring and control assembly for a wavelength stabilized optical system. The assembly includes a tunable laser and an adjustable wavelength selective filter whereby the signal is split selectively by wavelength of the signal into a reflected portion and a transmitted portion of said signal. At least one photodetector is used to monitor a beam of light as a function of wavelength of light emitted by the laser and the position of the wavelength selective filter.
Strasser and Wagener in U.S. Pat. No. 5,832,156, issued in 1998, describe a dispersive optical waveguide tap comprising a refractive index grating, coupling means and utilization means. Optical co-operation between the waveguide and the coupling means changes the guiding conditions, thereby directing the guided mode light into one or more radiation modes instead of transmission mode.
Jennings et al. in U.S. Pat. No. 5,712,942, issued in 1998, describe an optical communications system having distributed intelligence. Distributed intelligence interconnection modules optically connect one optical channel to another optical channel. The modules allow monitoring, testing and/or related activities of the overall optical communications system to be performed locally at the interconnection modules in an automatic and continuous manner. The interconnections each have actively intelligent microcontrollers thereon. A signal is tapped using an optical tap. The type of optical tap is not specified.
Corke et al. in U.S. Pat. No. 5,510,917, issued in 1996, describe optical communication monitoring and control. A fraction of signals being transmitted along an optical communications channel, at different signal carrier wavelengths, is tapped using an optical tap. A set of communication signals is transmitted using at least two distinct carrier optical wavelengths. The signals are demultiplexed and compared to a standard. Thereby the performance quality of the optical pathway is evaluated.
All of the above described methods and apparatus use fixed fiber optical signal splitters having fixed tap ratios that are capable of capturing a fraction of the optical signal. The fraction of the signal required varies with the monitoring function, for example: monitoring the power of a signal 1%, monitoring wavelength of a signal 3%, monitoring the quality of the signal 5%, and capturing encoded data from optical signal 10%. Of course, these are estimated values that vary depending with proximity of the monitoring apparatus to the transmission source or a transmission booster. It is not desirable to divert 10% of the signal to the monitoring apparatus if 1% will do, as the signal is weakened by whatever fraction of the signal is diverted to the monitoring apparatus. At the present time monitoring apparatus are, therefore, selected based upon specific monitoring requirements.